The Flex 3 should add HR monitoring, while keeping the form factor (including no screen). The Alta's are nice, but too bulky (and can't be used for swimming).
 
This community discussion sums it up pretty well:
 
https://community.fitbit.com/t5/Flex-2/Request-for-Flex-3/td-p/2937962
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		 
		
		
	
	
	
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